In recent years, a technology to form an integrated circuit integrally with a pixel portion on an inexpensive glass substrate for a flat panel display, that is what is called a System On Panel is attracting attentions. In that trend, a research regarding the formation of a large scale integrated circuit using a thin film semiconductor film is also advanced. It is quite important in designing a large scale integrated circuit such as a CPU to perform not only a simulation to evaluate only a logic function but also a simulation including data such as a propagation delay caused by wiring capacitance and capacitance (gate capacitance) formed between a gate electrode and an active layer of a transistor. In the case of an integrated circuit formed by using a thin film semiconductor film, in particular, a degree of integration is lower than that of an integrated circuit formed on a single crystalline silicon wafer, therefore, wiring capacitance is not easily suppressed, thus propagation delay time of a clock signal tends to be long. Therefore, it is essential for improving the yield and ensuring the frequency characteristics to determine accurate propagation delay time in the design phase and check the operation by simulation.
A semiconductor element formed by using a thin film semiconductor film, however, easily varies in characteristics as compared to a semiconductor element formed by using a single crystalline silicon safer. Therefore, there is a problem that propagation delay time of a clock signal caused by gate capacitance also easily varies. That is, propagation delay time of a clock signal caused by the gate capacitance cannot be accurately known until actually forming an integrated circuit. Therefore, the propagation delay time cannot be determined by simulation accurately. Thus, it is difficult to realize high frequency characteristics and high yield.
As for an integrated circuit using a single crystalline silicon wafer, on the other hand, a CPU which can ensure an operating frequency of about 2 GHz is, for example, put into a practical use and advanced in higher frequency. In future, it is expected that an integrated circuit which can ensure an even higher operating frequency is realized. However, in such a high frequency operation, a variation of propagation delay time caused by a variation in characteristics of semiconductor elements is a problem even in the case of using a single crystalline silicon wafer. That is, as an operating frequency rises, a ratio of a variation of the propagation delay time to a cycle of a clock signal increases. Therefore, it is difficult to realize high frequency characteristics and high yield similarly to an integrated circuit using a thin film semiconductor film.